1. Field of the Invention
This invention relates to a method and apparatus for forming a porous glass preform for use in preparing optical waveguide fibers.
2. Description of the Prior Art
Various techniques are known in the art for forming porous glass preforms. See, for example, Fujiwara et al., U.S. Pat. No. 4,135,901, Anderson et al., U.S. Pat. No. 4,136,828 (the "'828 patent"), Bachman et al., U.S. Pat. No. 4,203,553 (the "'553 patent"), Powers, U.S. Pat. No. 4,378,985 (the "'985 patent"), Berkey, U.S. Pat. No. 4,486,212 (the "'212 patent"), Powers, U.S. Pat. No. 4,568,370 (the "'370 patent"), Berkey, U.S. Pat. No. 4,684,384 (the "'384 patent"), and European Patent Publication No. 154,500.
The basic technique, as shown in the Berkey '212 patent, involves reciprocating a soot-producing burner along the length of a preform, while rotating the preform about its axis. A mandrel or bait rod, which is removed during subsequent processing of the preform, is normally employed to start the process. In some cases, a consolidated glass rod produced from a previously formed preform is used in place of the mandrel.
Over the years, modifications have been made to the basic technique to increase the efficiency with which preforms are produced. For example, a plurality of separately reciprocating burners, spaced at intervals around the periphery of a preform, has been used. Another improvement is illustrated in FIG. 1. As shown in this figure, instead of using a single reciprocating burner, a plurality of ganged burners 13a, 13b, and 13c are mounted on carriage 15 and moved in unison along the length of preform 17. In these ways, substantial increases in soot deposition rates have been achieved.
The use of ganged burners however does have its drawbacks. Specifically, the end portions of preforms produced with multiple burners have properties different from the central portion and thus those end portions cannot be used to produce fiber. Moreover, the length of the unusable portions increases as the number of burners is increased.
This effect is illustrated in FIG. 1, where the spacing between the first and last burners 13a and 13c is D and the distance through which carriage 15 is moved is L. As shown in this figure, when the carriage is at the left hand edge of the blank, a first necked-down area 19 is formed in the region which burner 13c does not reach, and a second necked-down are 21 is formed in the region which both burners 13b and 13c do not reach. Corresponding necked-down areas are formed on the right hand side of the preform.
As a result of these end effects, the usable length of the finished preform is only L-D. Attempts have been made to solve this problem by decreasing the distance between adjacent burners so as to decrease D. This approach has not been successful due to interference between the flames produced by adjacent burners. Accordingly, in the existing prior art reciprocating systems, only a limited number of burners, e.g., three burners, have been ganged together.
In addition to the basic method, a variety of other approaches have been used to produce porous preforms. One such approach involves longitudinal movement of the preform past a group of burners. The Powers '370 and '985 patents, referred to above, use this approach. In particular, these patents disclose collecting soot on a rotating member by moving the member past a series of oscillating burners.
Another approach for producing preforms involves using what is known in the art as a ribbon burner. Such burners have multiple, closely-spaced orifices each of which produces its own flame. In accordance with the ribbon burner technique, neither the burner nor the preform is moved longitudinally. Rather, the orifice spacings, burner configuration, and burner gas flows are chosen so as to produce a continuous soot sheet which deposits soot over the entire length of the preform. The Anderson et al. '828 and Bachman et al. '553 patents describe this approach.
A patent which is particularly relevant to the present invention is the Berkey '384 patent. This patent describes a system for producing preforms in which a plurality of burners are passed along the full length of the preform. In certain embodiments, the system is used to produce multiple preforms simultaneously, with the preforms being located along, for example, the sides of a square and with the burners being located inside the square and moving in a continuous loop parallel to the sides of the square.
In discussing problems with other approaches for forming preforms, the '384 patent mentions and dismisses the idea of using multiple burners and traversing each burner back and forth over a portion of the preform. Specifically, the patent states at column 2, lines 21-26, that if many burners are employed and each is traversed back and forth along only a segment of the entire preform, the soot buildup is not uniform throughout the entire length of the preform since all burners cannot provide precisely the same composition and amount of soot. Thus, the '384 patent specifically teaches away from the present invention.